Electronic vacuum pump including a sputter electrode



July 2, 1968 D, HALL 3,391,303

ELECTRONIC VACUUM PUMP INCLUDING A SPUTTER ELECTRODE Filed Jan. 25, 1965 FHAMENT POWER SUPPLY suaummou TARGET POWER sounca bPuTrE'R-- ON POWER F LAMENT PO WER $0M RCE N VENTOR SUB\.\MAT\ON 52mg- LE W15 0. HALL WER B 7 so u RCE I Afivlzwexs United States Patent 3,391,303 ELECTRONIC VACUUM PUMP INCLUDING A SPUTTER ELECTRODE Lewis D. Hall, 235 Ferne Ave., Palo Alto, Calif. 94306 Filed Jan. 25, 1965, Ser. No. 427,833 17 Claims. (Cl. 315-108) ABSTRACT OF THE DISCLOSURE A vacuum pump structure including a substantially single chamber housing a pair of opposed sputtering cathodes in which the pump walls serve as the anode. A sublimation structure is mounted in a special portion of the single chamber pump and in line of sight with the sputter cathodes to vapor deposit reactive metal on the sputtering cathodes.

This invention relates to vacuum pumps and, more par ticularly, to improvements in electronic vacuum pumps.

Electronic vacuum pumps in which reactive metals are employed to absorb gaseous molecules, and thereby reduce the pressure in an interconnected structure are presently well known. Some pumps employ a cold cathode sputtering technique in which electrons accelerated by a magnetic field collide with gas molecules to form positive ions and dissociated and metastable molecules. The positive ions sputter the cathode by dislodging gas absorbing material from its surface. The dislodged material is generally attracted to the anode and is deposited thereon. While being deposited, the material absorbs gas molecules, thereby reducing the pressure within the chamber of the pump.

Other electronic pumps employ a sublimation technique whereby a gas absorbing reactive metal is first sublimed, and thereafter condenses on a relatively large surface. After condensing, the reactive metal absorbs gas molecules within the pump structure, thereby decreasing the pressure within the pump as well as in an interconnected structure.

From experience with both types of electronic vacuum pumps, hereafter referred to as the sputter-ion pump and the sublimation pump, It is appreciated that each type has marked advantages and limitations as compared with the other. For example, compared with the short life of the sublimation pump, which generally requires some cooling, at sputter-ion pump, when operated properly, has a very long life and need not be cooled in order to perform satisfactorily for most applications. On the other hand, the structure of a sublimation pump, as well as the design of the sources of power for its operation, are much simpler than those necessary for a sputter-ion pump. Also, in a sublimation pump which has a high resistance to stalling, the pumping speeds for non-noble gases can be quite high, with the pump being easily scalable to the desired size.

Attempts have been made to produce a pump which operates on a combination of the sputter-ion and sublimation techniques. Though some attempts met with limited success, due to the strutural characteristics of prior art combination pumps, the performance of none of them measures up to the improved performance that a combination pump is expected to exhibit.

Accordingly, it is an object of the present invention to provide a new vacuum pump in which sputter-ion and sublimation pumping techniques are employed in a novel manner.

Another object of the present invention is the provision of a new electronic vacuum pump having a substantially single chamber in which sputter-ion and sublima- 3,391,303 Patented July 2, 1968 "ice tion pumping techniques are employed to produce improved pumping performance.

Another object of the invention is to provide a novel structure of a sputter-ion pump which is easily manufacturable and provides improved performance over prior art pumps.

Still another object of the invention is to provide an improved structure of a vacuum pump in which sputterion, sublimation or the combination of both techniques are employed to produce improved pumping capabilities.

Still a further object of the present invention is the provision of a pump structure which can be easily cooled, and which is accessible for replacement of internal components.

These and other objects of the invention are achieved by providing a vacuum pump structure which includes a substantially single chamber, with the pump wall serving as the anode of the sputter-ion portion of the pump. Whenever a sublimation pumping technique is employed together with the sputter-ion pumping, the sublimation structure is mounted in a special portion of the single chamber pump. A sublimation target is located, with -re' lation to the sputter-ion cathodes, so that sublimed metal is depositable directly onto the sputter cathodes. The sublimed material deposited on the cathodes covers positive ions which were previously buried therein, particularly noble gases and hydrogen, and prevents their being disinterred by subsequent bombardment. Thus, the life of the cathodes is greatly increased, and higher pumping speed is obtained for noble gases such as argon which in turn greatly increases the pumping speed of air.

The technique of directly depositing the sublimed metal on the cathodes of the sputter-ion portion of the pump eliminates the effect of argon instability, generally present in prior art pumps. In addition, the combined sputter-ion and sublimation pumping, in accordance with the teachings of the invention, enables the scaling of the pump to any desired speed. Furthermore, the pump can be started at roughing pressure without stalling, and can produce .a high vacuum without the aid of forepumping. Thus, the advantages of sputter-ion and sublimation pumping are realized in a single combination pump.

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself both as to its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawings, in which:

FIGURE 1 is a side cross-sectional view of a sputterion pump of the present invention;

FIGURE 2 is a top cross-sectional view of the sputterion pump of FIGURE 1;

FIGURE 3 is a side cross-sectional view of a combination sputter-ion-sublimation pump of the present invention;

FIGURE 4 is a side View of an improved sublimation target; and

FIGURE 5 is a side cross-sectional view of an improved sublimation pump of the present invention.

Reference is now made to FIGURES 1 and 2 wherein a vacuum pump 11 is shown comprising a substantially cylindrical metallic housing 12 having a length along its longitudinal axis designated by an arrow L, and a diameter designated by an arrow D. The interior surface of the wall of housing 12 which serves as the anode of the pump, is designated by numeral 13, and is shown electrically connected to a reference potential such as ground. A pair of disc-shaped cathodes 16 and 17, constructed of a reactive metal, are shown supported by conducting support members 18 and 19 which are insulatably coupled to the housing 12 by means of insulators 22 and 23. Thus,

3 members 18 and 19 also serve as terminals for connecting the cathodes 16 and 17 to external circuitry, such as a power supply 20. The cathodes 16 and 1'7 can alternatively be supported from a single member, as 18, with a shielded internal connection therebetween. However, such an arrangement is not as rigid mechanically as the arrangement shown in FIGURE 1.

The cathodes are mounted at substantially opposite ends of the housing 12 with their surfaces substantially perpendicular to the longitudinal axis of the housing 12. A magnet 25, generally comprising barium ferrite blocks, is positioned adjacent the housing 12 so that a magnetic field is produced in a direction parallel to the longitudinal axis of the housing. The pump housing 12 further includes a tubulation 26 having a flange 27 towhich a system to be evacuated (not shown) can be conveniently coupled;

From the foregoing description, it is appreciated that by connecting the cathodes by means of conducting members 18 and 19 to the power supply 20, which is assumed to provide negative potential with respect to ground to which the anode 13 is connected, the pump 11 will perform as a sputter-ion pump. Namely, a stray electron in the region between the anode 13 and either of the cathodes 16 and 17 will be attracted toward the positive (ground) anode. Assuming that the electron gains sufiicient kinetic energy, it may ionize any neutral gas molecule with which it may collide. The dissociated and metastable molecules which result from such collisions will be trapped by reactive material sputtered onto the surface of the anode. The anodes surface is continuously replenished by additional reactive material from the cathodes which are being sputtered by positive ions produced by the collisions. Thus, the absorption of the neutral molecules by the reactive surface of the anode helps to produce the desired vacuum. The advantages realized by the sputter-ion pump 11 shown in FIGURES 1 and 2 may further be increased by incorporating within the pump a sublimation assembly which is both structurally and electrically intercoupled thereto so as to greatly enhance the pumping performance thereof.

Reference is now made to FIGURE 3 in which a sublimation assembly 31 is shown coupled to an arrangement similar to that of pump 11, with like numerals designating like elements. As seen from FIGURE 3, the housing 12 includes an additional tubular member 32 and a flange 33 to which a support structure 34 of the sublimation assembly 31 is coupled. The structure 34 may be cylindrically shaped with one open end directed toward the hous- 1 ing 12. A reactive metal 35 which hereinafter will be referred to as the sublimation target, is shown supported by a conducting support member 37 which is electrically insulated from the structure 34 by means of an insulator 38. The member 3 7 has a portion thereof to serve as a terminal for connecting the sublimation target to an external sublimation target power source 40. In addition, the assembly 31 includes a heating filament 41 which is mounted on a pair of electrical conductors 42 and 43, adjacent the sublimation target 35. An insulator 44 insulatably couples the conductors 42 and 43 to the structure 34 so that portions of the conductors serve as terminals for coupling the filament to a source of current, such as power source 45. The current from source 45 beats the filament which.

bombards the sublimation target 35 with a stream of electrons, subliming the metal of which the target is constructed.

From the foregoing description, it is thus seen that the arrangement within housing 12 acts as a sputter-ion pump, whereas the sublimation assembly 31 provides a source of sublimed material. Experiments have shown that it is preferable to have the sublimation structure outside the portion of the pump housing which serves as the sputterion portion of the pump. Yet, it is important that a direct line-of-sight be provided between the sublimation target 35 and the cathodes 16 and 17. Both of these desired features are present in the arrangement shown in FIGURE 3 4;, in which target 35 is outside housing 12, yet is within a line-of-sight of cathodes 1s and 17.

As is known by those familiar with the art, the anode 13 is heated by electron bombardment, which occurs as part of the sputter-ion pumping. In addition, the anode heats up by thermal radiation from the sublimation target 35. Generally, the problems of cooling the anode are quite complex since in most prior art pumps, small anode cells are generally moun ed within the pumps which cannot be cooled by simple external means. However, in the pump of the present invention, the internal surface of housing 12 is used as the anode. Thus, the anode may easily be cooled by reducing the temperature of the housing 12 with an external cooling source such as air or water.

From the FIGURE 3, it should be appreciated that the structure of the pump of the present invention which includes the housing 12 and assembly 31 may be thought of as comprising a single cell or chamber with a positive-ground sputter-ion anode. The pump housing i at the anode potential. Thus, only the sputter cathode and the sublimation target, which is in a direct line-of-sight with the cathodes, are electrically isolated from ground potential. As a result, the intense discharge, which occurs during the starting phase of pumping, bombards only the reactive metal of the cathodes and thereby produces suflicient sputtering to provide satisfactory starting operation.

From FIGURE 3, it is further seen that even though the sublimation target 35 is not within the portion of the pump which acts as a sputter-ion pump, the target is in a direct line-of-sight with the sputter cathodes 16 and 17. Such structural relationship greatly enhances the performance of the pump, since the sublimed metal from the target covers ions, particularly of noble gases and hydrogen, which were previously buried in the cathodes 16 and 17. Consequently, the ions are prevented from being disinterred by subsequent ion bombardment of the cathodes, thereby increasing the speed at which the pump is capable of pumping noble gases. By increasing the pumping speed of noble gases, including argon which comprises about 1% of air, the pumping speed of air is greatly increased.

From the foregoing description of the vacuum pump of the present invention, it should be appreciated that by combining sputter-ion and sublimation pumping techniques in a single chamber wherein the sublimed target and the sputter cathodes are in a direct line-ofsight, the overall performance of the pump is greatly enhanced. In addition, the cooling problems which are ever present in sublimation pumps, are greatly minimzed by structurally using as the anode, the pump housing or casing so that the anode is conveniently cooled by externally cooling the housing.

It should also be appreciated by those familiar with the art that since the anode 13 is at ground potential, the sublimation target power source 40 has to supply a positive high voltage (negative ground) to the sublimation target 35, and a negative high voltage (positive ground) must be supplied by power supply 20 to cathodes 16 and 17. Preferably, both sources of power should be current-limited so that they could operate into a substantial short-circuit without damage. In addition, power source 40 is designed to provide a substantially constant voltage over its operating range in order to insure that if the emission current to the filament is turned off, the voltage of the sublimation target does not rise excessively, which otherwise may produce undesired arcing and corona. Similarly, the power supply 20 is designed to provide constant-wattage over the portion of its operating range near the high-current region. Such constantwattage characteristic prevents excessive overheating of the pump during the starting period of pumping or during heavy gas loads.

In the foregoing description, reference has been made to the cathodes 16 and 17 and the sublimation target as comprising a gas absorbing reactive metal. As is well known in the art, titanium is one of the metals often used for such purposes. Though titanium has been used, the teachings disclosed herein need not be limited to any particular reactive metal. For example, the cathodes 16 and 17 may be constructed of vanadium, and chromium may be used for the sublimation target 35.

In prior art sublimation pumps, one of the major problems concerns the construction of a proper target. Various techniques have been developed to support the reactive metal to be sublimed in a proper structure so that the reactive metal is properly sublimed. Generally, a tungsten rod which serves as the support structure, is used to support a solid rod of reactive metal such as titanium, or a titanium wire wrapped around it. Such sublimation targets, though used, are structurally unsatisfactory, especially when the heat produced by the filament is not well regulated. Whenever the temperature accidentally rises above a safe level, the titanium, instead of becoming gradually sublimed, starts to melt and flow on the tungsten rod and falls off, thus abruptly terminating the pumping operation.

Reference is now made to FIGURE 4 which is a side view of an improved sublimation target, such as target 35 (FIGURE 3). As seen, the target comprises a support structure, such as a rod 50, which may be constructed of tungsten. Rod 50 performs a function similar to that of support member 37 (FIGURE 3). Tungsten rod 50 supports alternate layers of a gas absorbing reactive metal such as titanium and a refractory metal such as. molybdenum, tantalum or tungsten. Layers 52, 54, and 56 represent the reactive metal, while layers 51, 53, 55 and 57 represent the refractory metal. The structural arrangement shown in FIGURE 4 is most effective in preventing the reactive material from flowing at high temperatures, even below melting. Even when the reactive metal softens, the surface tension thereof maintains the softened metal between the layers of the refractory metal. Thus, the limitations "of prior art sublimation targets due to accidental rise in temperature, are overcome by the novel sublimation target of the present incention. In one actual reduction to practice, a tungsten rod of 0.062 inch in diameter was effectively used to support alternate layers or sheets of titanium and molybdenum. The titanium sheets were 0.062 inch thick while very thin sheets of 0.005 inch molybdenum were used.

Reference is again made to FIGURE 3 wherein the sublimation target 35 is shown mounted adjacent the heating filament 41, with a direct line-of-sight therebetween. Such a filament-sublimation target arrangement is similar to those used in presently known sublimation pumps. Though the direct line-of-sight between the target and the filament may not always be disadvantageous, in some structural arrangements, it may be desirable to prevent the impingement of sublimed reactive metal of the target on the hot filament with which the metal may react. This may be accomplished by interposing a shield between the sublimation target and the hot filament in order to prevent sublimed metal of the target from impinging on the filament. The shield has the further important function of modifying the electric field configuration in such a way as to improve greatly the stability of electron flow between filament and target.

Reference is now made to FIGURE 5 which is a crosssectional side view of a single chamber sublimation pumping assembly 61. The assembly 61 may be combined with a sputter-ion assembly (FIGURE 1) to form a combination sputter-ionsublimation pump, or it may perform independently as an improved sublimation pump. As seen in FIGURE 5, the assembly 61 comprises a metallic chamber 62 connected to a reference potential such as ground with the interior wall 63 of the chamber serving as the anode. A sublimation target 64, constructed of a multilayer arrangement as hereinbefore described, is

mounted within the chamber, and connected to an external sublimation target power source 65 through insulator 66. Similarly, a filament 68 is mounted within the chamber 62 by means of conducting supports 71 and 72 which connect the filament to an external filament power source 73 through an insulator 74. In addition, a shield 75 consisting of a small sheet of a refractory metal, such as molybdenum, is interposed between the filament 68 and target 64 in order to block the direct line-of-sight therebetween. The shield 75 is supported by means of a conducting member 76 which may be electrically insulated from the chamber 62 by an insulator 77. The member 76 is preferably electrically connected to source 73, or to ground, in order to maintain the shield 75 at or near filament potential, so that the shield does not attract any electrons. The shield should not be greater than necessary to block the line-of-sight between the target 64 and the filament 68, in order not to require unduly large target voltages for adequate sublimation.

There has accordingly been described and shown herein an improved novel and useful electronic vacuum pump. One embodiment of the invention is a novel single chamber pump employing sputter-ion and sublimation pumping techniques. As hereinbefore described and shown, the sputter-ion portion, or the sublimation portion of the combination pump, may be used independently of one another. In addition, the sublimation portion of the combination pump or a separate sublimation pump may incorporate the novel sublimation target and/ or the shield hereinbefore described in order to greatly improve the pumping performance of the vacuum pumps taught herein. Also, such sublimation arrangements need not be limited to electron bombardment heating sources shown and described hereinbefore. Other heating techniques, such as resistance-heating, or induction-heating may similarly be employed.

It will be appreciated that those familiar with the art may make modifications and equivalents of the present invention as shown. These are to be considered as being within the scope and spirit of the invention. Therefore, all such modifications, equivalents and substitutions are deemed to fall within the scope of the appended claims.

What is claimed is:

1. A vacuum pump comprising:

an evacuable chamber having a wall defining a space adapted to contain gas molecules and including means for coupling said chamber to an evacuable system, said chamber forming the sole anode means of said pump;

means for electrically coupling said chamber so that substantially the entire wall of said chamber is at a first reference potential;

cathode means of a gas absorbent metal immovably located within said chamber;

means for electrically coupling said cathode means to a negative reference potential with respect to said first reference potential;

magnetic means for providing a magnetic field in a selected direction with respect to said evacuable chamber and said cathode means and of an extent such that at least the major portion of said electrode means lies within said magnetic field to ionize by electron collision some of said gas molecules whereby said ions disintegrate portions of said cathode means for absorbing gas molecules as said portions condense on said chamber wall; and

a sublimable target of a reactive metal for depositing some of said reactive metal on the wall of said chamber and on said cathode means as said target is sublimed.

2. A vacuum pump comprising a chamber having a wall thereof defining a space adapted to contain gas molecules, said chamber including two ends perpendicularly disposed with respect to a longitudinal axis thereof, and including means for coupling said chamber to an evacuable system; means for electrically coupling said chamber so that substantially the entire surface of the wall thereof is at a first potential; disc-like electrode means of a gas-absorbent metal immovably located adjacent either end of said chamber; means for electrically coupling said disclike electrode means to a negative potential with respect to said first potential; means for providing a magnetic field in a direction substantially parallel to the longitudinal axis of said chamber and extending throughout the space defined between said disc-like electrode means to sputter said disc-like electrode means whereby some of said gas molecules are absorbed by the sputtered metal condensing on the wall of said chamber; and a sublimation assembly coupled to said chamber comprising: a sublimation target of a reactive metal, located adjacent and outside the space defined between said disclike electrode means in a direct line-of-sight with substantially a major portion of said disc-like electrode means; means for electrically coupling said sublimation target to a positive potential with respect to said first potential and means for subliming the reactive metal of said sublimation target whereby at least a part of the sublimed metal is directly Sublimated on said disc-like electrode means.

3. A vacuum pump as recited in claim 2 wherein the ratio of the longitudinal axis of said chamber to an axis perpendicular thereto is not less than one half.

4. A vacuum pump as recited in claim 2 wherein said sublimation target comprises a first plurality of layers of reactive metal supported on a conducting member, and a second plurality of layers of refractory metal, each interposed between layers of said reactive metal, whereby the surface tension of said reactive metals substantially maintains each of said first plurality of layers between a pair of said second plurality of layers.

5. A vacuum pump as recited in claim 2 wherein said sublimation target comprises a support memberya plurality of alternating layers of a reactive metal and a refractory metal, with each layer of said reactive metal supported between two layers of said refractory metal.

6. A vacuum pump as recited in claim 2 wherein said means for subliming include a source of electrons which is disposed and arranged to cause the electrons to directly bombard said reactive metal.

7. A vacuum pump as recited in claim 6 further including shield means interposed between said source of electrons and said sublimation target for substantially pre venting sublimed metal from said sublimation target from impinging on said source of electrons.

8. A pump for producing a vacuum therein by sputtering with ions a reactive metallic cathode thereof, which when deposited on an anode absorbs gas molecules, thereby reducing the pressure within said pump, and by subliming a reactive metal which is in a direct line-of-sight with respect to said cathode so that sublimed metal is deposited on said cathode, comprising an evacuable chamber having a wall defining a space adapted to contain gas molecules, and including means for coupling said chamber to an evacuable system; at least one disc-like cathode of a gas absorbent reactive metal located adjacent one end of said chamber perpendicular to a longitudinal axis thereof; magnetic means for producing a magnetic field in a direction parallel to the longitudinal axis of said chamber; a first source of electrical potential; means for connecting said chamber and said cathode to said first source of electrical potential with the wall of said chamber being at a higher potential than said cathode; forsputtering said cathode with ions produced by collisions between accelerating electrons and gas molecules, whereby portions of the reactive metal of said sputtered cathode absorb gas molecules while deposited on the wall of said chamber; a sublimation support structure connected to said chamber for extending the space defined by the wall of said chamber to define an auxiliary sublimation space therein; a sublimation target of a reactive metal located within said sublimation support structure adjacent the space defined by the wall of said chamber so as to be in a direct line-of-sight with said disc-like cathode; a second source of electrical potential; means for coupling said sublimation target and said chamber to said second source of electrical potential. whereby said sublimation target is at a substantially higher potential with respect to the first potential of the wall of said chamber; and heating means for heating the reactive metal of said sublimation target for subliming at least portions thereof, whereby at least some of the sublimed metal is deposited on said disc-like cathode, being in substantially a direct line-of-sight with said sublimation target so as to replenish the reactive metallic surface of said cathode.

9. A vacuum pump as recited in claim 8 wherein said heating means comprise filament means for heating the reactive metal of said sublimation target by electron bombardment, and shield means interposed between said filament means and said sublimation target for blocking sublimed reactive metal of said sublimation target from impinging on said filament means.

10. A vacuum pump as recited in claim 9 wherein said sublimation target comprises a support member and a plurality of alternating layers of a reactive metal and a refractory metal, with each layer of said reactive metal supported between two layers of said refractory metal.

11. A vacuum pump as recited in claim 8 wherein said sublimation target comprises a support member and a plurality of alternating layers of a reactive metal and a refractory metal, with each layer of said reactive metal supported between two layers of said refractory metal.

12. A sublimation target for an ion-getter vacuum pump comprising:

a support member of a refractory metal; and

a plurality of alternate layers of reactive and refractory metals supported by said support member, said layers of refractory metals being selected as thin as consistent with providing support to the immediately adjacent relatively thicker layers of reactive metals.

13. In a pump for producing a vacuum in a chamber by subliming a gas-absorbent metal of a sublimitation target with a stream of bombarding electrons from an electron source so that when the sublimed metal is vapor deposited within said chamber, gas molecules therein are absorbed by the deposited metal, the improvement with a stream of bombarding electrons from an electron source so that when the sublimed metal is vapor deposited within said chamber, gas molecules therein are absorbed by the deposited metal, the improvement comprising a sublimation target having: a support member; and a plurality of alternating layers of a gas-absorbent reactive metal and a refractory metal, whereby substantially every layer of said reactive metal is supported by said support member between two layers of said refractory metal.

15. In a pump for producing a vacuum in a chamber by subliming a gas-absorbent metal of a sublimation target with a stream of bombarding electrons from an electron source so that when the sublimed metal is vapor deposited within said chamber, gas molecules therein are absorbed by the deposited metal, the improvement comprising a sublimation targent having: a support member, and a plurality of alternating layers of a gas-absorbent reactive metal and a refractory metal, whereby substantially every layer of said reactive metal is supported by said support member between two layers of said refractory metal; shield means disposed within said chamber for blocking sublimed metal from impinging on said electron source; and means for maintaining said shield means at substantially the potential of said electron source to prevent electrons from being attracted thereto.

16. A sputter-ion vacuum pump comprising:

a chamber having a Wall thereof defining a space adapted to contain gas molecules, substantially said entire wall serving as and forming the sole anode electrode of said pump;

disc-like cathode electrodes of a gas-absorbent reactive metal insulatably coupled to said chamber immovably located adjacent the ends thereof with the major axis of each of said disc-like cathode electrodes perpendicular to the longitudinal axis of said chamber;

means for producing a magnetic field parallel to the longitudinal axis of said chamber and extending throughout the space defined between said disc-like cathode electrodes; and

means for connecting said chamber and said disc-like cathode electrodes to a source of electrical potential to sputter said cathode electrodes with positive ions produced by electron bombardment of gas molecules, whereby the sputtered reactive metal absorbs gas molecules after condensing on said wall to increase the vacuum within said space.

17. A sputter-ion vacuum pump as recited in claim 16 wherein the ratio of the longitudinal axis of said cylindrical chamber to an axis along a cross-section of said chamber perpendicular to the longitudinal axis thereof, is at least equal to one-half, and wherein said source of electrical potential is a source of positive ground potential with said chamber being at ground potential and said cathode electrodes at a negative potential with respect thereto.

References Cited UNITED STATES PATENTS 2,006,081 6/1935 Anderson 313-211 X 2,845,567 7/1958 Geiger 313-217 X 3,107,044 10/1963 Brubaker 32433 X 3,152,752 10/1964 Vanderslice 23069 3,176,906 4/1965 Redhead 230-69 3,235,170 2/1966 Thorensen 230-69 2,477,279 7/1949 Anderson 313355 X 3,244,969 4/ 1966 Herb et al 315-111 X FOREIGN PATENTS 1,075,272 2/ 1960 Germany.

JAMES W. LAWRENCE, Primary Examiner.

STANLEY D. SCHLOSSER, Examiner.

R. L. IUDD, Assistant Examiner. 

